An automobile door is produced typically by fitting a door inner panel and a door outer panel together. A window, a window drive device, an audio speaker, a handle, and other various components are to be attached to the automobile door. For set-in of these components, a space must be made between the door inner panel and the door outer panel. In order to make the space, for example, vertical walls are made in the door inner panel. Also, when the door is closed, the inside of the automobile must be sealed by the door. In order to seal the inside of the automobile, for example, a stair is provided in the vertical walls of the door inner panel. The stair of the vertical walls is caused to face a pillar or the like of the vehicle body, and this ensures tight sealing of the inside of the automobile.
The door inner panel is produced by press working of a steel plate. Such a door inner panel generally has a complicated shape, and accordingly, in producing such a door inner panel, it is necessary to deform a steel plate largely. In this case, the door inner panel after the press working may have cracks, wrinkles, and the like. In order to suppress generation of cracks, wrinkles, and the like, usually, a soft steel plate with high workability is used as the material of the door inner panel.
However, the soft steel plate has low strength. Accordingly, a door inner panel formed of a soft steel plate has low strength. Therefore, a reinforcing member (for example, a beltline reinforcement, a door impact beam, and the like) is often attached to the door inner panel.
Door inner panels are disclosed, for example, in Japanese Patent Application Publication No. 2007-296953 (Patent Literature 1), Japanese Patent Application Publication No. 2008-94353 (Patent Literature 2) and Japanese Patent Application Publication No. 2013-112133 (Patent Literature 3).
The door inner panel disclosed in Patent Literature 1 includes a beltline reinforcement. The beltline reinforcement is attached to a beltline portion of the door inner panel and extends in the front-rear direction of the vehicle. Patent Literature 1 states that the beltline reinforcement bears a collision load applied to the vehicle in the vehicle front-rear direction, which effectively decreases the bending moment acting on the beltline portion.
In the door inner panel disclosed in Patent Literature 2, when a collision load is applied from a side of a vehicle, the door inner panel and the beltline reinforcement contact with each other, and a load absorbing portion of the door inner panel is deformed. Patent Literature 2 states that the load absorbing portion absorbs a part of a load applied in the thickness direction of the door inner panel, which ensures rigidity of the door inner panel.
In the side door disclosed in Patent literature 3, the rear end portion and the front end portion of a beltline reinforcement formed by hot stamping have strength and rigidity which are lower than those of the body of the beltline reinforcement. Accordingly, when a collision load is applied from the front side of the vehicle, the rear end portion of the beltline reinforcement is plastically deformed, and the contact area between the rear end portion of the beltline reinforcement and the center pillar increases. Patent Literature 3 states that the deformation of the rear end portion of the beltline reinforcement allows absorption of collision energy.